Field of the Invention
The subject-matter of the invention relates to a method for producing a sintered honeycomb body from thin layers of steel sheet resistant to high-temperature corrosion. Such honeycomb bodies are used in particular in exhaust systems of motor vehicles as catalyst support bodies and/or as substrates for adsorber material.
International Patent Disclosure WO 96/08336 discloses a method for producing a metallic structure, in particular a catalyst support body, from at least partly structured wound, intertwisted or laminated layers of steel sheet. In which the layers of steel sheet are subjected to a thermal treatment for forming a metallic bond of the layers of steel sheet with or without auxiliary substances.
An auxiliary substance may be a soldering powder, which is applied to the regions of the layers of steel sheet to be soldered. Published, Non-Prosecuted German Patent Application DE 29 24 592 A1 discloses various methods of applying solder.
According to International Patent Disclosure WO 96/08336, the formation of connecting regions between the layers of steel sheet may also take place without auxiliary substances. For instance, by a thermal treatment being carried out at a temperature below the melting point of the layers of steel sheet, so that the layers of steel sheet are sintered to one another.
Published, Non-Prosecuted German Patent Application DE 29 47 694 A1 also discloses that the sintering operation is simplified by the layers of steel sheet being provided with a metal coating before the sintering. The sintering technique has so far not become established in practice for the forming of honeycomb bodies, in particular for catalyst support bodies.
In applications of honeycomb bodies in exhaust systems, it is often important that they can be rapidly heated up. A reduction in the mass is particularly important for this purpose and can be achieved by the use of relatively thin layers of steel sheet. In the case of known catalyst support bodies, the sheet thickness is usually approximately 40 to 50 xcexcm. In the case of layers of steel sheet of which the thickness is less than or equal to 35 xcexcm, it has been established that the quality of the catalyst support bodies can be ensured only with difficulty if the layers of steel sheet are connected to one another by soldering methods. Accumulations of solder may lead to alloying at the connecting locations, which reduces the resistance to high-temperature corrosion.
It is accordingly an object of the invention to provide a method for producing a sintered honeycomb body that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
Against this background, the invention is based on the object of specifying a method for producing a high-quality metallic honeycomb body which contains layers of steel sheet of a thickness of less than or equal to 35 xcexcm. At the same time, the method is to be commercially advantageous. A correspondingly produced, non-soldered honeycomb body is also to be provided.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing a metallic honeycomb configuration. The method includes forming a honeycomb body by at least one of stacking and winding layers of steel sheet containing chromium and aluminum. A thickness of each of the layers of steel sheet being less than or equal to 35 xcexcm and at least some of the layers of steel sheet are structured layers of steel sheet so that the honeycomb body has channels formed therein through which a fluid can flow. The honeycomb body is then introduced into a tubular jacket. Connections are formed between neighboring layers of steel sheet by a thermal treatment in which the honeycomb body is heated up to over 1100xc2x0 C. The honeycomb body is then cooled down, with an average temperature transient of at least in each case 25 K/min.
The rapid heating up in comparison with conventional soldering or sintering methods is very advantageous in the first instance because the production time is reduced as a result. However, rapid heating up also has advantages with respect to the chemical processes at the surface of the layers of steel sheet, since oxidation processes impairing the connection are reduced, while the formation of, for example, carbides, in particular chromium carbides, which enhance the connections, is encouraged. Since the thin sheets have a low thermal capacity, they can also be heated up more rapidly, so that holding times at certain temperatures for evening out the temperature profile in the oven can be avoided to the greatest extent. The entire thermal bonding process can in this way be shortened to 90 minutes or less. The average temperature transient is understood as meaning the average change in temperature per unit of time during the entire heating-up operation, from the initial temperature in the oven to the highest temperature, or the corresponding change in temperature per unit of time during the cooling-down operation.
In accordance with a preferred feature of the invention, there is the step of performing the thermal treatment by annealing the layers of steel sheet of the honeycomb body.
In accordance with another feature of the invention, there is the step of heating the honeycomb body in the range of 1150xc2x0 C. to 1190xc2x0 C. during the thermal treatment.
It is also favorable in the case of the method according to the invention to allow the thermal treatment to proceed in a vacuum or at least a partial vacuum with a defined atmosphere, in order to avoid unwanted oxidation effects and selectively influence the surface properties of the honeycomb body.
It is preferred for the layers of steel sheet to be provided with a rolling oil of a high carbon content, in particular in the form of graphite powder, before the forming of the honeycomb body, with the effect that a layer of carbon for enhancing the sintering process is formed in the thermal treatment. When the steel sheets are produced with the desired thickness by cold rolling, a rolling oil is always used in any case, but in conventional processes for producing honeycomb bodies usually has to be removed before or during the thermal treatment. According to the invention, however, a rolling oil of a kind which during thermal treatment forms residues which essentially contain carbon is chosen, since this encourages the sintering processes. By adding graphite, for example, preferably in the form of fine powder, to the rolling oil, this effect can be further intensified. Graphite also has the advantage that it acts as a lubricant, which is of great advantage in the production of structures, in particular corrugations, on very thin sheets, as are used for honeycomb bodies according to the invention.
Furthermore, according to a preferred embodiment of the invention, the number of connections produced in individual regions can be influenced by selective introduction of stresses or by stress relief, in particular in the case of honeycomb bodies containing a multiplicity of layers of steel sheet running in an approximately spiral or involute form.
It should be pointed out that the rapidity of the heating up, the type of rolling oil and the selective introduction of stresses or stress relief are in each case advantageous on their own, but also in any desired combination, for the sintering process.
The thermal treatment is followed by a cooling-down step. This may be carried out in a vacuum or under inert gas. Any remains of carbon, which could cause problems later when coating with a catalytically active material or an adsorber material, can be eliminated during the cooling-down operation by selective feeding in of an inert-gas/oxygen atmosphere. In this case, the carbon remains burn and a defined oxide layer forms, improving the adherence of later coatings. In a preferred embodiment the honeycomb body is cooled down at an average temperature transient of 30 K/min.
The set object is also achieved by a honeycomb body, formed by stacking and/or winding layers of steel sheet containing chromium and aluminum. At least some of which are structured layers of steel sheet touching at touching locations, so that the honeycomb body has channels through which a fluid can flow. The honeycomb body is surrounded by a tubular jacket. The layers of steel sheet have a thickness less than or equal to 35 xcexcm and at least some of the touching locations having at least one connecting region that contains carbides, in particular chromium carbides. Such a honeycomb body is mechanically very stable and resistant to high-temperature corrosion, even in the case of sheet thicknesses below 35 xcexcm or even below 25 xcexcm.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing a sintered honeycomb body, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.